The present invention relates generally to a measurement method and apparatus, and particularly to a measurement method and apparatus for measuring an optical characteristic of a projection optical system that projects a pattern of a reticle (mask) to a substrate.
A projection exposure apparatus has so far been employed which uses the lithography technology to manufacture fine semiconductor devices such as a semiconductor device, e.g., an IC and an LSI, an imaging device, e.g., a CCD, a display device, e.g., a liquid crystal panel, a magnetic head. A projection exposure apparatus transfers a pattern of a reticle (mask) onto a substrate such a wafer via a projection optical system. Since the exposure apparatus is required to precisely transfer a pattern of a reticle to a substrate with a specified magnification, it is important to use a projection optical system that has an excellent imaging characteristic and a restrained aberration. Especially in recent years, finer processing to the semiconductor device progresses, and a pattern to be transferred has become sensitive to an aberration of an optical system. Accordingly, there is a demand to highly precisely measure a projection optical system's optical characteristic (e.g., a wavefront aberration) with the projection optical system included in an exposure apparatus. In addition, in order to improve productivity and economic efficiency, a simple, fast, and inexpensive measurement is also important.
Conventionally, a projection optical system's wavefront aberration has been measured by actually exposing a reticle pattern onto a wafer, and observing its resist image using such a means as a scanning electron microscope (“SEM”). This conventional measurement method has a problem in a poor reproducibility of measurement due to a difficult SEM operation and errors in a resist application and a development.
In order to rapidly and accurately measure a projection optical system's wavefront aberration, it is desirable to use an interference method, rather than using the conventional measurement method that exposes a pattern on a resist for evaluation. However, the conventional interference method that uses a Fizeau interferometer, a Twyman-Green interferometer, or the like makes an overall system's structure complex, thus implying a large-size and high-cost problem. Thus, it is difficult to mount the interferometer on an exposure apparatus, and the conventional interference method is not viable.
Therefore, an exposure apparatus is proposed that has a comparatively simple interferometer such as a point diffraction interferometer (hereinafter called a “PDI”), a line diffraction interferometer (hereinafter called an “LDI”), and the like. For example, see Japanese Patent Application, Publication No. 2004-273748.
However, a measurement of a wavefront aberration using the PDI or LDI does not consider a polarization state of a light incident upon an image side measurement pattern (or an image side slit), and causes a measurement error in measuring a wavefront aberration of an optical system having a large numerical aperture (“NA”). For example, when a high-NA optical system such as the projection optical system in the exposure apparatus is measured using the LDI, a width of the image side slit in its shorter direction becomes smaller than the wavelength of the incident light (or the exposure light). It is known that an amplitude or phase of a diffracted light from an opening smaller than the wavelength changes depending on polarization direction of the incident light and a direction of the opening, if the incident light is a linearly polarized light. Further, when a linearly polarized light enters a slit that is as large as or smaller than the incident light's wavelength, a diffracted light's amplitude or phase changes depending on a ratio between the linearly polarized light's sx-axis component and sy-axis component, where the sy-axis is an axis parallel to the slit's longitudinal direction, and the sx-axis is an axis parallel to its shorter direction.
FIG. 12 shows phase distributions (wavefronts) of a diffracted light from a slit when the linearly polarized light of the incident light is parallel to the sy-axis (TE) and is parallel to the sx-axis (TM). FIG. 13 shows amplitude distributions of a diffracted light from a slit when the linearly polarized light of the incident light is parallel to the sy-axis (TE) and is parallel to the sx axis (TM). The phase distribution and the amplitude distribution are calculated by using an electromagnetic field analysis (finite difference time domain (“FDTD”) method). Referring to FIG. 12, a maximum of 45 mλ of a phase difference is seen in the wavefront of the TE-mode incident light and the wavefront of the TM-mode incident light. Further, referring to FIG. 13, the amplitude of the TM mode incidence is about one half of that of TE mode incidence.
The wavefront measurement using the LDI (hereinafter called “LDI measurement”) measures a wavefront using a pair of orthogonal slits. For example, assume that the incident light is a linearly polarized light parallel to the X-axis, and the LDI measurement uses a slit parallel to the X-axis (hereinafter called “Y slit”) and a slit parallel to the Y-axis (hereinafter called “X slit”). A coordination system is set such that an up-and-down direction of the apparatus is the Z-axis, a depth direction is the Y-axis, and a direction orthogonal to the Z-axis and Y-axis is the X-axis. In this case, a Y-axis directional diffracted wavefront from the Y slit is the wavefront labeled by the TE in FIG. 12, and an X-axis directional diffracted wavefront from the X slit is the wavefront labeled by the TM in FIG. 12.
The LDI's reference wavefront is calculated by using the Y-axis directional wavefront information of a diffracted wavefront from the Y slit, and the X-axis directional wavefront information of a diffracted wavefront from the X slit. Accordingly, as shown in FIG. 12, if a spherical equivalent differs between the diffracted wavefront from the X slit and the diffracted wavefront from the Y slit, an error corresponding to a cos 2θ component will occur in the combined reference wavefront. Thus, the polarized incident light causes an error in the LDI measurement, because a difference in diffracted wavefronts from the pair of slits becomes an error of the reference wavefront.